Natural killer T (NKT) cells are lymphoid cells which are distinct from mainstream T cells, B cells and NK cells (Arase et al., 1992, Proc. Nat'l Acad. Sci. USA, 89:6506; Bendelac et al., 1997, Annu. Rev. Immunol., 15:535). These cells are characterized by co-expression of NK cell receptors and semi-invariant T cell receptors (TCR) encoded by Vα14 and Jα281 gene segments in mice and Vα24 and JαQ gene segments in humans. The activation of NKT cells in vivo promptly induces a series of cellular activation events leading to the activation of innate cells such as natural killer (NK) cells and dendritic cells (DC), the activation of adaptive cells such as B cells and T cells, the induction of co-stimulatory molecules and the abrupt release of cytokines such as interleukin-4 (IL-4) and interferon-γ (IFN-γ) (Burdin et al., Eur. J. Immunol. 29: 2014-2025, 1999; Carnaud et al., J. Immunol., 163: 4647-4650, 1999; Kitamura et al., J. Exp. Med., 189: 1121-1128, 1999; Kitamura et al., Cell Immunol., 199: 37-42, 2000; Aderem et al., Nature, 406: 782-787, 2000). In addition, activated NKT cells can themselves bring about killing mediated by Fas and perforin. The full activation cascade can be recruited by the engagement of NKT TCR. Alternatively, powerful T-helper-cell type 1 (Th1) functions can be selectively triggered by cytokines such as interleukin-12 (IL-12) released by infected macrophages or DC. These functions are believed likely to be correlated with the important role of NKT cells in conditions such as autoimmune diabetes, rejection of established tumours or the prevention of chemically induced tumours (Yoshimoto et al., 1995, Science, 270: 1845; Hammond et al., J. Exp. Med., 187: 1047-1056, 1998; Kawano et al., 1998, Proc. Natl. Acad. Sci. USA, 95: 5690; Lehuen et al., J. Exp. Med., 188: 1831-1839, 1998; Wilson et al., Nature, 391: 177-181, 1998; Smyth et al., J. Exp. Med., 191: 661-668, 2000). Finally, NKT cells are thought to contribute to antimicrobial immunity through their capacity to influence the Th1-Th2 polarization (Cui et al., J. Exp. Med., 190: 783-792, 1999; Singh et al., J. Immunol., 163: 2373-2377, 1999; Shinkai et al., J. Exp. Med., 191: 907-914, 2000). These cells are therefore implicated as key effector cells in innate immune responses. However, the potential role of NKT cells in the development of adaptive immune responses remains unclear.
Glycolipids are molecules typically found in plasma membranes of animal and plant cells. Glycolipids contain an oligosaccharide which is bonded to a lipid component. Sphingoglycolipids are complex glycolipids which contain ceramide as the lipid component. One class of sphingoglycolipids is alpha-galactosylceramides (α-GalCer), which contain D-galactose as the saccharide moiety, and ceramide as the lipid moiety. α-GalCer is a glycolipid originally extracted from Okinawan marine sponges (Natori et al., Tetrahedron, 50: 2771-2784, 1994).
It has been demonstrated that α-GalCer can activate NTK cells both in vitro and in vivo. α-GalCer has been shown to stimulate NK activity and cytokine production by NKT cells and exhibit potent antitumor activity in vivo (Kawano et al., 1997, Science 278: 1626-9; Kawano et al. 1998, supra; Kitamura et al. 1999, supra). Kitamura et al. (1999, supra) demonstrated that the immunostimulating effect of α-GalCer was initiated by CD40-CD40L-mediated NKT-DC interactions. As the immunoregulatory functions of α-GalCer were absent in both CD1d-1- and NKT-deficient mice, this indicates that α-GalCer has to be presented by the MHC class I-like molecule CD1d.
CD1 is a conserved family of non-polymorphic genes related to MHC that seems to have evolved to present lipid and glycolipid antigens to T cells and in this way participates in both an innate and an adaptive pathway of antigen recognition (reviewed by Park et al., Nature, 406: 788-792, 2000; see also Calabi et al., Eur. J. Immunol., 19: 285-292, 1989; Porcelli et al., Annu. Rev. Immunol., 17: 297-329, 1999). The CD1 family comprises up to five distinct genes (isotypes) that can be separated into two groups on the basis of sequence homology. Group 1, which comprises CD1a, CD1b, CD1c and CD1e, is present in humans but absent from mouse and rat. Group 2, which includes CD1d, is found in all species studied so far, including humans.
CD1 isotypes are expressed selectively by antigen-presenting cells such as dendritic cells (DCs), macrophages and subsets of B cells, but apart from CD1d expression in hepatocytes they are generally not expressed in solid tissues (Porcelli et al., supra; Bendelac et al., Annu. Rev. Immunol., 15: 535-562, 1997).
α-GalCer is recognized in picomolar concentrations by mouse and human CD1d-restricted lymphocytes that express a semi-invariant TCR and exert potent effector and regulatory functions (Kawano et al., 1997, supra). CD1d/α-GalCer complex is, in turn, recognized by the antigen receptors of mouse Vα14 and human Vα24 natural killer T (NKT) cells (Bendelac et al., Science, 268: 863-865, 1995; Bendelac et al., Annu. Rev. Immunol., 15: 535-562, 1997; Park et al., Eur. J. Immunol., 30: 620-625, 2000).
α-GalCer has been demonstrated to activate murine NKT cells both in vivo and in vitro, upon binding to CD1d (Kawano et al., 1997, supra; Burdin et al., 1998, J. Immunol., 161:3271-3281), and in human NKT cells in vitro (Spada et al., 1998, J. Exp. Med., 188:1529-1534; Brossay et al., 1998, J. Exp. Med. 188:1521-1528). For example, α-GalCer was shown to display NKT-mediated anti-tumor activity in vitro by activating human NKT cells (Kawano et al., 1999, Cancer Res., 59:5102-5105).
In addition to α-GalCer, other glycosylceramides having α-anomeric conformation of sugar moiety and 3,4-hydroxyl groups of the phytosphingosine (such as α-glucosylceramide [α-GlcCer], Galα1-6Galα1-1′Cer, Galα1-6Glcα1-1′Cer, Galα1-2Galα1-1′Cer, and Galβ1-3Galα1-1′Cer) have been demonstrated to stimulate proliferation of Vα14 NKT cells in mice, although with lower efficiency (Kawano et al., Science, 278: 1626-1629, 1997, supra). By testing a panel of α-GalCer analogs for reactivity with mouse Vα14 NKT cell hybridomas, Brossay et al. (J. Immunol., 161: 5124-5128, 1998) determined that nearly complete truncation of the α-GalCer acyl chain from 24 to 2 carbons does not significantly affect the mouse NKT cell response to glycolipid presented by either mouse CD1 or its human homolog.
It has been also demonstrated that in vivo administration of α-GalCer not only causes the activation of NKT cells to induce a strong NK activity and cytokine production (e.g., IL-4, IL-12 and IFN-γ) by CD1d-restricted mechanisms, but also induces the activation of immunoregulatory cells involved in acquired immunity (Nishimura et al., 2000, Int. Immunol., 12: 987-994). Specifically, in addition to the activation of macrophages and NKT cells, it was shown that in vivo administration of α-GalCer resulted in the induction of the early activation marker CD69 on CD4+ T cells, CD8+ T cells, and B cells (Burdin et al., 1999, Eur. J. Immunol. 29: 2014; Singh et al., 1999, J. Immunol. 163: 2373; Kitamura et al., 2000, Cell. Immunol. 199:37; Schofield et al., 1999, Science 283: 225; Eberl et al., 2000, J. Immunol., 165:4305-4311).
Various α-GalCer compounds have been shown in the prior art. U.S. Pat. No. 5,780,441 describes mono- and di-glycosylated α-GalCer compounds of the following structure:
wherein R1 is H or
    R2 is H,
    R3 and R6 are H or OH, respectively,    R4 is H, OH or
    R5 is H or
    x is an integer from 19 to 23; and    R7 is —(CH2)11—CH3, —(CH2)12—CH3, —(CH2)13—CH3, —(CH2)9—CH(CH3)2, —(CH2)10—CH(CH3)2, —(CH2)11—CH(CH3)2, —(CH2)11—CH(CH3)—C2H5,wherein at least one of R1, R2, R4 and R5 is a glycosyl moiety.
The compounds are disclosed for use as antitumor agents, as bone marrow cell-proliferation treating agents, and as immunostimulating agents.
Recently, α-GalCer molecules have also been shown to have activity against viral diseases. Kakimi, J. Exp. Med. 192: 921-930 (2000) discloses that natural killer (NKT) cells in the liver of hepatitis B virus (HBV) transgenic mice were activated by a single injection of α-GalCer, thereby inhibiting HBV replication. α-GalCer has also been shown to be effective against microbial infections. Gonzalez-Asequinaloza, Proc. Natl. Acad. Sci. USA 97: 8461-8466 (2000) discloses that the administration of α-GalCer inhibits the development of malaria parasites, resulting in strong antimalaria activity.
α-GalCer has also demonstrated inhibition of the onset and recurrence of autoimmune type I diabetes. Sharif, Nature Medicine 7: 1057-1062 (2001) demonstrates that activation of NKT cells by α-GalCer protects mice from type I diabetes and prolongs the survival of pancreatic islets transplanted into newly diabetic mice. See also Hong, Nature Medicine 9: 1052-1056 (2001). Sharif also demonstrated that when administered after the onset of insulitis, α-GalCer and IL-7 displayed a synergy, which is believed to be due to the ability of IL-7 to render NKT cells fully responsive to α-GalCer.
α-GalCer has also demonstrated antifungal activity. Kawakami, Infection and Immunity 69: 213-220 (2001) demonstrates that upon administration to mice, α-GalCer increased the serum level of gamma interferon, resulting in inhibition of the fungal pathogen Cryptococcus neoformans. 
α-GalCer analogs have also demonstrated effectiveness against autoimmune diseases. Miyamoto, Nature 413: 531-534 (2001) describes use of α-GalCer analogs which induce TH2 bias of autoimmune T cells by causing natural killer T (NKT) cells to produce IL-4, leading to suppression of experimental autoimmune encephalomyelitis.
A synthetic analog of α-GalCer, KRN 7000 (2S,3S,4R)-1-O-(α-D-galactopyranosyl)-2-(N-hexacosanoylamino)-1,3,4-octadecanetriol, can be obtained from Pharmaceutical Research Laboratories, Kirin Brewery (Gumna, Japan) or synthesized as described in Morita et al., J. Med. Chem., 1995, 38: 2176-2187.
KRN 7000 has the structure:

KRN 7000 has been shown to display activity against tumors in mice. Kobayashi, et al., Oncol. Res. 7:529-534 (1995). In particular, KRN 7000 has been shown to be effective in preventing cancer metastasis. See, e.g., Nakagawa, Canc. Res. 58, 1202-1207 (1998) (KRN 7000 effective in treating liver metastasis of adenocarcinoma colon 26 cells in mice). KRN 7000 is also described in Kobayashi et al., 1995, Oncol. Res., 7:529-534, Kawano et al., 1997, Science, 278:1626-9, Burdin et al., 1998, J. Immunol., 161:3271, and Kitamura et al., J. Exp. Med., 1999, 189: 1121, and U.S. Pat. No. 5,936,076.
Importantly, in addition to its ability to stimulate immune responses, recent human trials have shown that α-GalCer is not cytotoxic in humans. See Shimosaka et al. Cell Therapy: Filling the gap between basic science and clinical trials, First Int'l Workshop 2001, abstract pp. 21-22. Other studies have demonstrated that α-GalCer, independently of its dosage, does not induce toxicity in rodents and monkeys (e.g., Nakagawa et al., 1998, Cancer Res., 58: 1202-1207), although a recent study showed the transient elevation of liver enzyme activities immediately after α-GalCer treatment in mice, suggesting a minor liver injury (Osman et al., 2000, Eur. J. Immunol., 39: 1919-1928).
However, most mammals, including humans, have abundant amount of α-galactosidase, an enzyme which digests α-GalCer by catalyzing the degradation of α-D-galactoside bonds. As a result, α-GalCer has a short half-life, and therefore its in vivo therapeutic effect may be reduced.
Recently, it has been shown that the activity of α-GalCer can be modified through formation of a truncated sphingosine chain. The modified α-GalCer is effective in treating autoimmune encephalomyelitis in mice. Miyamato et al., Nature 413:531-534 (2001).
Applicants have now discovered α-GalCer analogs which have improved stability in vivo over α-GalCer.
Applicants have also discovered α-GalCer analogs which have improved therapeutic efficacy over α-GalCer.